1. Field of the Invention
This invention relates to motor control circuits for controlling the starting and stopping of servo motors, and more particularly to a solid state motor control circuit which approximates a dynamic braking function.
2. Description of the Prior Art
Many control systems employ small servo motors controlled by proportional controllers to operate functional devices for maintaining a specific system parameter at a set point value. For example, in heating and air conditioning systems, one or more motors may be selectively operated to move valves or dampers to correct for deviations of the temperature in a conditioned area from set point values. Whenever the temperature rises above or falls below the set point value, the proportional controller generates an error signal which is applied to a motor control circuit for operating a servo motor causing its associated valve or damper to move to effect compensation for the temperature change. The motor may also drive a potentiometer to provide a feedback signal to the proportional controller to indicate when the error has been corrected for.
In such applications, accurate control of the starting and stopping of the motor is essential to provide the desired correction for the deviation from the set point while avoiding overshoot. Accordingly, when the set point is reached, dynamic braking is normally used to stop the motor.
In applications in which the controller generates digital logic level signals for effecting energization of the motor, a control relay is generally used as an interfacing element between the motor and the control logic to isolate the motor supply from the digital logic circuitry. The control relay is energized by relatively low level signals provided by the control logic and when energized connects power at a higher level to the motor via its contacts. A further relay controls the polarity of the volage applied to the motor to select direction of travel for the motor shaft. When correction for the error signal has been achieved, the relays are deenergized, disconnecting power from the motor.
In known circuits, dynamic braking is provided using the control relay contacts to connect both terminals of the motor to ground when the control relay is deenergized. In such motor control circuits, the high currents induced in the motor circuit when the relay drops out, results in arcing at the relay contacts. This may result in welding or deterioration of the contacts, affecting reliability of the control circuit. While it is possible to modify the relays to increase reliability, this adds considerable cost to the control system.